PROJECT SUMMARY/ABSTRACT (DESCRIPTION) More than 2 in 3 adults in the US are overweight or obese [1, 2]. Both conditions, and especially obesity, contribute to many health conditions including diabetes, heart disease, and cancers [3-5]. National costs due to obesity are high: $342 billion dollars (2013) in medical costs, which is 28% of all adult healthcare spending [6]. One form of behavioral intervention for weight loss is telephone-delivered coaching [7-9]. Telephone coaching has clear advantages: high population level reach and individually tailored training from a coach [10-12]. Telephone coaching for weight loss reaches over 1.2 million US adults per year [7, 12-14]. However, a critical barrier to progress in the field is that standard behavioral therapy (SBT) telephone coaching interventions have small effect sizes that are rarely evaluated against active treatment controls. Needed now are telephone coaching programs with the potential to boost weight loss success over and above SBT. We propose a new behavioral intervention for telephone coaching: Acceptance and Commitment Therapy (ACT) [15]. Unlike SBT, ACT interventions address the fundamental challenge of weight loss: overeating in response to internal (e.g., stress) and external (e.g., high calorie foods) cues [16-19]. ACT for weight loss addresses disinhibition by focusing on (1) increasing willingness to experience physical cravings, emotions, and thoughts that cue eating and impede physical activity while (2) making healthy diet and physical activity choices guided by deeply held values [16, 20]. While ACT has been applied to many behaviors and in a variety of delivery modalities [21-24], for weight loss it has only been tested in RCTs for in-person interventions [23, 25-30]. Dr. Bricker?s team recently conducted a multi-step design process that yielded an ACT telephone coaching protocol. We tested the protocol in a pilot RCT (N = 105), comparing it with telephone coaching SBT. Compared to SBT, ACT participants had greater success on the 10% or more weight loss main outcome, at both the 3- and 6-month follow-up. Building on these encouraging results, we propose to conduct a fully powered randomized controlled trial of ACT telephone coaching (n = 199) versus SBT telephone coaching (n = 199), in order to determine if telephone coaching ACT: (1) has significantly higher weight loss at 12 months post randomization, and (2) has 12-month weight loss on the main outcome (and secondary outcomes) mediated by these ACT-consistent psychological processes: (a) acceptance of food cravings, (b) acceptance of discomfort from physical activity, (c) mindful eating, and (d) values guided motivation to change. We will explore whether the 12-month weight loss main outcome for ACT, versus SBT, differs by these baseline factors: (a) age, (b) sex, (c) race/ethnicity, (d) BMI, (e) depression, (f) anxiety. If successful, telephone coaching ACT will offer a more effective, broadly scalable weight loss treatment?thereby making a high public health impact.